This invention relates to tubular members designed to have fluid flow therethrough, and, more particularly, to tubular members of the type that are intended primarily for utilizaton in a boiler, and which embody means operable for exercising control over the movement of the fluid in the tubular member.
It is widely known that a major operating component of any conventionally constructed steam generatng system is the boiler. Likewise, it is widely known that it is in the boiler that the generation of steam is actually effected. In this regard, the aforesaid generation of steam is commonly accomplished as a consequence of the passage of water through a multiplicity of tubular members, i.e., tubes, during which passage the water is sufficiently heated so as to cause it to change state, i.e., to change from a liquid to a vapor.
Within a heat transferring or vapor, i.e., steam generating system, there exists an infinite number of conditions relating to temperature, primary and secondary fluid movements, and structural configurations that influence the performance thereof. Accordingly, it is both desirable and beneficial to have the capability to design into such systems precise and accurate control of the fluid movement therewithin so as to maximize the efficiency of the system.
The present invention is directed to a boiler tube that is provided on the inside surface thereof with physical characteristics that are required thereby in order to insure the existence of a proper fluid movement therethrough so that the heat transfer rate to the fluid medium is maximized. To this end, the tube bore surface is made uneven such as, for instance, by deforming the existing tube material, or by affixing additional material thereto as through the welding thereof, or by depositing weld metal thereon. Such modifications in or additions to the material that constitutes the tube bore surface is intended to function to provide a turbulence to the fluid thereby tending to eliminate a temperature gradient within the fluid at any cross sectional plane taken across the tubular area. In addition, this exercise of control over the fluid motion within the tube is also intended to function to prevent the formation of vapor pockets or vapor layers adjacent to the tube inner wall surface that operate to produce an adverse effect on the heat transfer properties of the medium.
More specifically, it is obviously important that boiler tube failure be avoided in steam generating systems. Moreover, insofar as concerns the matter of boiler tube failure, one known cause thereof is overheating of the tube. Furthermore, it is known that an inefficient transfer of heat through the tube wall to the water, i.e., fluid medium flowing therewithin can lead to the tube overheating. The reference here to an inefficient transfer of heat through the tube wall is meant to encompass the situation wherein the accomplishment of the desired heat transfer process is impeded by the presence of so-called nucleate boiling, i.e., wherein stagnation of steam bubbles that function in the nature of insulation impedes the passage of the heat through the tube wall to the water flowing therewithin.
By way of a summary, the condition which is sought to be avoided here in an effort to minimize the susceptibility of the tubes to become overheated as a result of nucleate boiling, is that wherein there exists within the tube a laminar flow of water or steam. As used herein, the term laminar flow is meant to refer to a stream line or viscous flow of the fluid axially of the tube. Namely, it is desired to effect the breaking up of such laminar flow in the tube.
As the water flows through the tube, the outer layer of the water, i.e., the layer of water in closest proximity to the inner wall of the tube, becomes heated by the heat being transmitted through the tube wall. As a consequence thereof, the outer layer of water changes to steam. During this process of changing to steam, the first change, which the outer layer of water undergoes, is the formation therein of steam bubbles. Mention has previously been had herein of the fact that steam bubbles act as an insulation. Consequently, unless the steam bubbles, which are being formed in the outer layer of water, are made to mix, they will, in essence, remain stationary, i.e., stagnate and take on the attributes of an insulative film, thereby causing localized hot spots to develop along the tube wall. Moreover, such hot spots, in turn, can cause overheating of the tube, and ultimately lead to tube failures. Additionally, unless they are made to mix, the steam bubbles, by virtue of their insulative capability, will also function to prevent further heating of the core of water, which is passing rapidly through the center of the tube in the form of laminar flow, the latter term being employed herein as defined above.
Thus, from the preceding, it should now be readily apparent that in order to achieve the rapid and efficient transfer of heat through the tube walls to the water flowing therewithin, there exists a need to provide some form of means that would be operative to effect the breaking up of the laminar flow of water through the tube. Namely, some such form of means is needed to effect the mixing of the outer layer of water and thereby also the steam bubbles entrained therein with the core of water flowing through the central region of the tube. As noted previously above, one such form of means, which has been employed heretofore in the prior art, to achieve a controlled internal disruption of the flow of water through a boiler tube has involved the usage of ribbing, i.e., rifling, on the internal surface of the boiler tube.
As regards the nature of the existing prior art teachings relating to the matter of providing boiler tubes with rifled inner wall surfaces, reference may be had, by way of exemplification, for purposes of obtaining a familiarity therewith to the following U.S. Pat. Nos. 3,088,494; 3,213,525; 3,272,961; 3,289,451 and 3,292,408. More specifically, U.S. Pat. No. 3,088,494, which issued to P. H. Koch, et al on May 7, 1963, is directed to providing a vapor generating tube that has its interior wall formed with helical lands and grooves, which are proportioned and arranged in a particular predetermined fashion. In accord with another such exemplary prior art teaching, U.S. Pat. No. 3,213,525, which issued to W. M. Creighton, et al on Oct. 26, 1965, is directed to a method of forming an internal rib in the bore of a tube wherein material is removed from the inner tube wall by means of a cutting operation to form the subject ribbing. A still further example of these prior art teachings can be found in U.S. Pat. No. 3,272,961, which issued to L. A. Maier, Jr., et al on Sept. 13, 1966 and wherein a method and apparatus are taught for making ribbed vapor generating tubes and in accordance with which a rib is deposited on the inside surface of a tube through the use of a welding process. U.S. Pat. No. 3,289,451, on the other hand, which issued to P. H. Koch, et al on Dec. 6, 1966, is directed to a method and apparatus for forming internal helical ribbing in a tube wherein the internal ribbing is formed by means of a cold drawing operation. Finally, U.S. Pat. No. 3,292,408, which issued to J. R. Hill on Dec. 20, 1966, is directed to a method of forming internally ribbed tubes wherein the tube is provided with an asymmetrical helical groove so as to facilitate removal of the forming tool from the tube.
Notwithstanding the existence of the afore-described prior art teachings, a need has nevertheless been shown to exist for a boiler tube that embodies a new and improved form of rib design. More specifically, the boiler tubes produced through the employment of prior art methods are known to be adversely characterized by the fact that they each suffer from certain notable disadvantages. For instance, a major disadvantage of boiler tubes produced by the methods and apparatus for effecting the formation of ribbed boiler tubes that have been known here-to-date resides in the invariableness of the rib design, which the boiler tube is made to embody. That is, there is an inherent inflexibility associated with the use of prior art methods and apparatus insofar as concerns effectuating variations in the configuration of the rib design that is to be formed in the surface of the tube inner wall.
Namely, as noted previously herein, nucleate boiling can lead to the development of localized hot spots that, in turn, cause overheating and ultimately boiler tube failures. To minimize the establishment of such localized hot spots in boiler tubes stemming from the existence of nucleate boiling, it has been proposed by the prior art to provide ribbing, i.e., rifling, on the inner wall surface of the tube. Unfortunately, however, the methods and apparatus known in the prior art heretofore for effectuating the making of such rifled tubing render it difficult to enable significant variations in pattern configuration to be implemented for purposes of compensating for variations in the location of potential hot spots along the inner walls of the tubes. That is, existing methods and apparatus are limited to the utilization of fixed patterns, such that each boiler tube, irrespective of the location it occupies in the boiler, i.e., its relative exposure to external sources of heat, is necessarily provided with the same pattern of rifling, even though from a heat transfer standpoint, it may be desirable to vary the pattern as between locations within the same tube, as well as between tubes in the same boiler.
By way of exemplification in this regard, particular attention is directed to U.S. Pat. No. 3,272,961 to which reference has previously been had hereinbefore. This patent contains a teaching of providing a boiler tube with rifling in the form of a weld deposit. The method and apparatus as taught therein, however, are disadvantageously characterized by their total lack of flexibility in effecting adjustments in the rifled pattern that is being formed in a boiler tube to compensate for providing the boiler tube with different heat transfer characteristics in various locations along the length thereof. Namely, in accord with the teachings of U.S. Pat. No. 3,272,961, the implementation of the formation of a rifled pattern in a tube is predicated upon the creation of a pattern that comprises a repeat of the same rifling configuration for the entire length of each individual boiler tube. Moreover, not only are changes in pattern of rifling as between different locations in the same tube difficult to effect with the apparatus described in the aforesaid U.S. Pat. No. 3,272,961, but also, it is difficult therewith to effect changes in pattern of rifling as between different tubes, wherein it is desired to have them embody individually different heat transfer characteristics. Principally, this is because to effect such changes requires the establishment of completely different relationships between the components, i.e., tube, welding means, etc., from those which these components bear one to another in order to effectuate the formation in a boiler tube of a given rib design. Namely, these components must have different relationships one to another for each different pattern of rifling, i.e., rib design, with which it is desired to provide a boiler tube.
In summary, in order to effect the exercise of control over the motion of the fluid flowing through a boiler tube, reliance is had to the use of a variety of forms, shapes, sizes and/or patterns that are suitably provided on the tube bore surface. More specifically, such control is effected through the employment of means, which, for purposes of the discussion herein, is characterized by being a rib, irrespective of the shape which the latter may embody, or whether it is continuous or non-continuous. Most importantly, the exact configuration which such a rib formed on the tube bore surface is made to embody is a variable that is dependent on the type of fluid medium, the fluid pressure, the fluid flow rate, the tube size, the temperature gradient within the tube wall in both a circumferential and a longitudinal direction, as well as other variables. A consideration of these factors for purposes of providing the inner tube wall with the physical characteristics that are required in order to insure a proper fluid movement therein so as to maximize the heat transfer rate to the fluid medium can lead to the utilization of a rib design wherein the parameters applicable thereto are selected from variations which are possible in the angle that the ribbing bears to the lateral axis of the tube; variations which are possible in the spacing between individual ribs of the ribbing; whether the ribbing is continuous or non-continuous; whether the ribbing is provided on only one internal side wall of the tube or on more than one internal side wall thereof; and the particular dimensions of the rib in terms of the specific heighth, the specific width, the specific radii and the specific angles that individual portions of a given rib embodies.
For example, variations in the angle that the ribs bear to the lateral axis of the tube and/or variations in the spacing between individual ribs in a given rib design offer a high degree of flexibility in compensating for the potential existence of localized hot spots. Namely, through the selection of the proper parameters from the aforesaid variables, it is possible to select a rib design wherein provision is made for the creation of the desired fluid flow properties for each increment of tube length so as to effect a realization of the proper fluid movement for maximizing the heat transfer rate along the entire length of the tube. It is to be noted that, for purposes of achieving a maximization of the heat transfer rate, it is conceivable that it could be deemed necessary to utilize a rib design wherein variations exist therein in each increment of tube length. Apart from the types of variations mentioned at the outset of this paragraph, the need to obviate the existence of localized hot spots may dictate the use of a rib design that is discontinuous, i.e., wherein the rib design is provided only in areas of the tube inner surface that are exposed to a high heat flux, and/or a rib design that is provided on only one side wall of the tube, i.e., on the side wall of the tube that corresponds to the heat source side thereof. Finally, note is taken of the fact that various rib shapes may be employed. In this regard, the rib that is utilized may be symmetrical in shape while in other instances, it may be deemed advisable to employ other rib shapes which offer directional flow features that can be advantageous in regard to pressure drop considerations. Also, the film coefficient of heat transfer can be improved through the proper selection of the shape of the rib inasmuch as the former is known to be dependent upon the direction of fluid flow relative to the heating surface. As with the spacing and the pitch of the ribs, the heighth, width, radii and angles of the various rib shapes can be controlled to fulfill predetermined design criteria. In summary, in accord with the present invention, it has been recognized that each rib shape and rib size will provide an individual flow motion and pressure drop, thereby offering an infinite range of fluid motion control for design selection to provide the desired individual flow characteristics for each incremental section of a heat transfer system.
It is, therefore, an object of the present invention to provide a new and improved form of tubular member of the type intended to have fluid flow therethrough.
It is another object of the present invention to provide such a tubular member, which embodies means operable for exercising control over the movement of the fluid in the tubular member.
It is still another object of the present invention to provide such a tubular member wherein control over the movement of the fluid therewithin is effected by providing the inner surface of the tubular member with ribbing, the latter embodying a variable configuration, the parameters of which are selected on the basis of the ability thereof to fulfill predetermined design criteria, i.e., the design criteria that will enable the tubular member to possess the fluid flow properties for each increment of length thereof required to achieve a maximization of the heat transfer rate to the fluid.
A further object of the present invention is to provide such a tubular member embodying on the inner surface thereof variable ribbing wherein one of the parameters that determines the nature of the design of the ribbing and which is a variable, is the angle which the ribs of the ribbing bear to the lateral axis of the tubular member, i.e., the pitch of the ribbing.
A still further object of the present invention is to provide such a tubular member embodying on the inner surface thereof variable ribbing wherein one of the parameters that determines the nature of the design of the ribbing and which is a variable is the spacing that exists between the individual ribs that collectively serve to comprise the ribbing.
Yet another object of the present invention is to provide such a tubular member embodying on the inner surface thereof variable ribbing wherein one of the parameters that determines the nature of the design of the ribbing and which is a variable is whether the ribbing is continuous or discontinuous.
Yet still another object of the present invention is to provide such a tubular member embodying on the inner surface thereof variable ribbing wherein one of the parameters that determines the nature of the design of the ribbing and which is a variable is whether the ribbing is provided on only one longitudinally extending side wall of the tubular member, or on more than one side wall thereof.
Yet a further object of the present invention is to provide such a tubular member embodying on the inner surface thereof variable ribbing wherein one of the parameters that determines the nature of the design of the ribbing and which is a variable is the shape of the individual ribs in terms of the heighth, width, radii and angles thereof.